The invention relates to probe cards for testing integrated circuits, and more particularly to probe cards having optical fibers.
Probe card assemblies generally include a probe card which is a printed circuit board having electrically conductive traces for carrying signals from contact pads formed on the integrated circuit. The probe card is interfaced with a programmed computer that generates test signals and senses responses from the integrated circuit. Electrically conductive probes are connected to the traces in the probe card and are arranged to be placed in electrical contact with the contact pads on the integrated circuit. Some assemblies also include an additional layer to transition from the low density probe card to a higher density of probes.
Problems recognized in the design of probe card assemblies include the uneveness of the contact pads, fatigue of probes, and misalignment of the probes caused by repetitive use. As integrated circuits become denser, the separation of contact pads is reduced and the challenges relating to the physical characteristics of the probe are increased. For example, as the probes become smaller, materials having higher conductivity must be used to compensate for the loss in probe size.
The increasing clock rate of integrated circuits is also creating design challenges for probe card assemblies. In particular, present circuits often run faster than 200 MHz, while probe card assemblies are designed to run much slower. An apparatus that addresses these problems is therefore desirable.
In various embodiments, the invention is a probe card having one or more optical fibers. Time-correlated single-photon-counting (TCSPC) techniques are used in one embodiment to detect a state change in a test structure of a circuit under test. Using optical fibers instead of traditional electrically conductive probes supports capture of device state changes that would be undetectable with traditional probes. In addition, the ability to detect a signal with an optical fiber is not dependent on establishing a good electrical contact between the fiber and a contact pad. Thus, positioning an optical fiber in order to detect a desired signal is not dependent on application of a suitable force to the fiber.